1. Field of the Invention
This invention relates broadly to apparatus and methods for investigating subsurface earth formations. More particularly, the present invention relates to borehole tools and methods which use a combination of fluid injection techniques and resistivity measurements for quantifying formation characteristics such as permeability, relative permeability, and skin factors. For purposes herein, the term "borehole" when utilized by itself or in conjunction with the word "tool" is to be understood in its broadest sense to apply to cased and uncased boreholes and wells.
2. State of the Art
The determinations of permeability and other hydraulic properties of formations surrounding boreholes such as relative permeability and skin factors are very useful in gauging the producibility of formations, and in obtaining an overall understanding of the structure of the formations. For the reservoir engineer, permeability and relative permeability are generally considered fundamental reservoir properties, the determinations of which are at least equal in importance with the determination of porosity, fluid saturations, and formation pressure. Indeed, determinations of relative permeabilities to oil and water are crucial for forecasting oil recovery during water flooding or natural water drives. The economic viability of a reservoir therefore depends upon the nature of these saturation dependent permeabilities.
Before production, when obtainable, cores of the formation provide important data concerning permeability. However, cores are difficult and expensive to obtain, and core analysis is time consuming and provides information about very small sample volumes. In addition, cores, when brought to the surface may not adequately represent downhole conditions. Thus, in situ determinations of permeability over the length of the borehole are highly desirable.
Suggestions regarding in situ determination of permeability via the injection or withdrawal of fluid into or from the formation and the measurement of pressures resulting therefrom date back at least to U.S. Pat. No. 2,747,401 to Doll (1956). The primary technique presently used for in situ determination of permeability is the "drawdown" method where a probe of a formation testing tool is placed against the borehole wall, and the pressure inside the tool (e.g., at a chamber) is brought below the pressure of the formation, thereby inducing fluids to flow into the formation testing tool. By measuring pressures and/or fluid flow rates at and/or away from the probe, and processing those measurements, determinations regarding permeability are obtained. These determinations, however, have typically been subject to large errors. Among the reasons for error include the fact that liberation of gas during drawdown provides anomalous pressure and fluid flow rate readings, and the fact that the properties of the fluid being drawn into the borehole tool are not known accurately. Another source of error is the damage to the formation (i.e., pores can be clogged by migrating fines) which occurs when the fluid flow rate towards the probe is caused to be too large. See, e.g., Ramakrishnan et al., SPE 22689 (1991).
More recent patent disclosures of permeability testing tools include U.S. Pat. No. 4,742,459 to Lasseter, and U.S. Pat. No. 4,860,581 to Zimmerman et al. (both of which are assigned to the assignee hereof) which further develop the draw-down techniques. The Zimmerman et al. patent mentions that in the drawdown method, it is essential to limit the pressure reduction so as to prevent gas liberation. In order to prevent gas liberation, Zimmerman et al. propose a flow controller which regulates the rate of fluid flow into the tool.
Additional progress in in situ permeability measurement is represented by U.S. Pat. Nos. 5,269,190 and 5,247,830, (both of which are assigned to the assignee hereof, and incorporated by reference in their entireties herein). In U.S. Pat. No. 5,269,190, borehole tools, procedures, and interpretation methods are disclosed which rely on the injection of both water and oil into the formation whereby endpoint effective permeability determinations can be made. In U.S. Pat. No. 5,247,830, methods are disclosed for making horizontal and vertical-permeability measurements without the necessity for measuring flow rate into or out of the borehole tool. These inventions advance the art significantly. However, even with the improvements in permeability measurement techniques, the accuracy and scope of the information obtained is not to the level desired. In particular, in the formation fluid sampling tools, only a limited number of samples may be obtained which can be analyzed. Thus, the locations from which the samples are taken must be well chosen. Further, even if sampling of formation fluids is not desired, but measurements are taken via drawdown and/or injection and measuring, it will be appreciated that each procedure is time-consuming. Thus, it is desirable to gain large amounts of information from each procedure, and again location of the tool in the borehole is critical as is the quantity and quality of the data accumulated. For example, it might be desirable to take the vertical permeability in a portion of a formation which crosses a bed boundary or a fracture, or alternatively to avoid such a situation. To cross a bed boundary or fracture, accurate location of the tool is required such that one probe or sensor lies on one side of the bed boundary or fracture while the other probe or sensor lies on the other side of the bed boundary. Similar accuracy is required to avoid straddling a boundary or fracture if such is desired.
While bed boundary locations are determinable and thin beds are locatable via the use of other well established tools such as the FMS (Formation Micro Scanner--another mark of the assignee hereof, details of which are found in Ekstrom, M. P. et al., "Formation Imaging With Microelectrical Scanning Arrays"; The Log Analyst; Vol. 28, No. 3, May-June 1987), and other tools (both impedance and current injection tools), it will be appreciated that this information obtained from a previous investigation of the borehole must be correlated with the depth of the permeability tool being run in the borehole at the time for a proper setting of the permeability tool. The tool depth is typically determined by monitoring the cable from which the tool is hung. However, because of the stretching and twisting of the cable, among other things, the exact location and orientation of the tool vis-a-vis the formation is never as exact as desired.
Some of these problems are overcome by the integrated permeability measurement and resistivity imaging tool set forth in co-owned U.S. Pat. No. 5,335,542, which is hereby incorporated by reference herein in its entirety. In U.S. Pat. No. 5,335,542, a tool having probes with electrodes and means for fluid withdrawal and/or injection are provided for making an investigation of the formation. As fluid is withdrawn or injected into the formation, the fluid pressure of the formation is obtained, and electromagnetic data is obtained by the electrodes. The electromagnetic and fluid pressure data are then processed using various formation and tool models to obtain relative permeability information, endpoint permeability, wettability, etc.
While the tool and method of co-owned U.S. Pat. No. 5,335,542 is believed to be effective in providing important relative permeability and other information, it will be appreciated that in order to gather information from which the desired determinations are made, the borehole tool must be in contact with the formation. Thus, the data gathering process is time consuming and data is limited to specific locations, although information regarding other locations can be generated from the data obtained at the specific locations. In addition, while some depth of investigation is obtained, the interpretation does not extend to a reservoir length scale.
As mentioned above, the skin (also called "skin factor" or "skin damage") of a well is another important variable in the production of a well. During the drilling of a well, the mudcake can invade the formation and alter the sandface, and hence the permeability of the formation adjacent the borehole. In addition, during production, fines in the produced fluid can move into the pores of the formation adjacent the borehole, thereby reducing the effective permeability of the formation. While it is known to shut down production and conduct a test which maps the pressure in the wellbore over time in order to assess skin-damage to the wellbore, it will be appreciated that the known test only provide a single value for the entire wellbore, while only portions of the wellbore may be damaged. Thus, if wellbore cleaning is attempted using acid, the acid may travel into the clean non-damaged areas of the formation, while skin damage correction is not productively accomplished.